The invention relates to an imaging apparatus and method for acquiring panoramic images of a very large visual field from a single viewpoint, at substantially uniform resolution, and at standard video rate.
A camera with a large field of view (the extent of the scene captured in the image by the camera) capable of acquiring seamless panoramic images of substantially the entire space in front of the camera at substantially uniform resolution is highly desirable in many applications such as tele-conferencing, surveillance and navigation. Substantially entire space in front here refers to the substantially the hemisphere in front of the camera, without gaps. (An analogously large field of view of substantially the entire space behind the camera, in addition to the front view, is also desirable, to achieve a larger field of view, or example, a combined front and rear spherical field of view with gaps.) Substantially uniform resolution here refers to the level of uniformity delivered by conventional nonpanoramic cameras, and in what follows, will be referred to simply by xe2x80x9cuniform resolution.xe2x80x9d It is also desirable to acquire the entire panoramic image from a single viewpoint and in real time (e.g., for 3D object modeling and display). Real time here means at standard video rates delivered by conventional video cameras, e.g., 30 frames/second. The field of view of a conventional digital camera is typically limited by the size of the sensor and the focal length of the lens, and the image resolution is limited by the number of pixels on the sensor. For example, a typical 16 mm lens with ⅔xe2x80x3 CCD sensor has a 30xe2x80x3xc3x9723xe2x80x3 field of view, and has a resolution of 640xc3x97480 pixels. Many efforts have been made to acquire panoramic images with large field of view. These have succeeded in achieving various subsets of the desired properties: large field of view, gap-free field of view, uniform resolution, a single viewpoint and real-time acquisition. A summary of these methods is presented in the next paragraph.
The past methods of panoramic and omnidirectional image acquisition fall into two categories: dioptric methods, where only refractive elements (lenses) are employed, and catadioptric methods, where a combination of reflective and refractive components is used. Typical dioptric systems include: camera cluster methods where each of a cluster of conventional cameras points in a different direction and together the cameras cover all different directions; fisheye methods where a single, conventional camera acquires a wide field of view image through a fisheye lens; and rotating camera methods where a conventional camera pans to generate mosaics, or a camera with a non-frontal, tilted sensor pans around an axis through its viewpoint to acquire panoramic images with all objects in focus. The catadioptric methods include: curved mirror methods where a conventional camera captures the scene as reflected off a single non-planar mirror, or mirror pyramid methods where multiple conventional cameras image the scene as reflected off the faces of a planar right mirror-pyramid. The dioptric camera clusters can achieve uniform resolution across a wide field of view at video rate. However, typically the cameras in the cluster do not share a unique viewpoint so there are gaps or overlaps between scene spaces covered by adjacent cameras, and therefore, it is impossible to seamlessly combine individual images to form a panoramic view without arbitrary blending at the image borders. The cameras with fisheye lens are able to deliver large field of view images at video rate, but suffer from low resolution, irreversible distortion for close-by objects, and changing viewpoints for different portions of the field of view. The rotating cameras deliver high-resolution over a wide field of view via panning, as well as omni-focus when used in conjunction with non-frontal imaging, but they have limited vertical field of view. Furthermore, because they sequentially capture different parts of the field of view, moving objects may be imaged incorrectly. The cameras that use a parabolic- or a hyperbolic-mirror to map an omni-directional view onto a single sensor are able to achieve a single viewpoint at video rate, but the resolution of the acquired image is limited to that of the sensor used, and further, it varies with the viewing direction across the field of view. Analogous to the dioptric case, this resolution problem can be resolved by replacing the simultaneous imaging of the entire field of view with panning and sequential imaging of its parts, followed by mosaicing of the images, but at the expense of video rate.
One of the past efforts, to which the present invention is most closely related, uses a right mirror-pyramid with vertex angle of 90 degrees and a set of cameras associated with the pyramid faces. Each camera is located in a plane containing the pyramid vertex and parallel to the pyramid base, it looks toward the pyramid base in a direction parallel to the pyramid axis, it is associated with one of the pyramid faces, and it captures the part of the scene reflected off its associated face. The apparent or virtual optical centers of all cameras coincide or nearly coincide at a common point along the axis of the pyramid. The virtual viewing directions of all cameras are perpendicular to the pyramid axis and the images they obtain comprise a 360-degree strip around the pyramid perpendicular to its base. The height of the strip is the same as the vertical visual field of the cameras. In another related invention, an additional camera is placed at the location of the virtual optical center looking toward the pyramid base along the axis of the pyramid. In order to mosaic the image captured by this additional camera with those captured by the remaining cameras, the additional camera needs a wide-angle lens since it must cover a disproportionately large field of view around the axis. Images from all cameras are combined to yield a panoramic image of the overall field of view. This panoramic image has the following characteristics: the 360-degree strip around the pyramid axis has a smaller size than the size of the visual field required to be captured by the additional camera, and consequently, the additional camera has to use a wide-angle lens; the entire panoramic image is captured from a single or nearly single viewpoint; and the panoramic image acquisition occurs in real time.
Therefore, one drawback of the prior art is that it fails to provide an apparatus or method for acquiring a panoramic image which shows a large field of view at uniform resolution.
In view of the foregoing, it is an object of the present invention to overcome these and other drawbacks of the prior art.
Specifically, it is an object of the invention to provide a method and apparatus for acquiring panoramic images from a single viewpoint of the scene around the viewing direction in front of the camera (to be called the front axial direction), but for a field of view (to be called the front field of view) which is nearly hemispherical about the direction of viewing and free of gaps.
It is another object to provide uniform resolution across the entire field of view.
It is another object to extend the nearly hemispherical front field of view of the first object towards being nearly spherical.
In order to accomplish a part of these and other objects of the invention, there is provided an imaging apparatus as described in the following. The apparatus consists of two main parts. The first part is mainly concerned with imaging the front near hemisphere part of the overall field of view and the second part is mainly concerned with imaging the rear near hemisphere part of the overall field of view. In the rest of the document xe2x80x9cnear hemispherexe2x80x9d will be referred to simply by xe2x80x9chemispherexe2x80x9d for brevity. The first part includes a right cylinder having a plurality of planar faces and a polygonal cross section, each face associated with one of the edges in the polygonal cross section. The outer surface of the cylinder is reflective. This first part of the apparatus also includes a set of conventional cameras placed around the right cylinder, one camera per face of the cylinder. Each camera captures a part of the scene reflected off its corresponding face. All cameras are placed so that the mirror images of their physical optical centers coincide at a single virtual optical center within the cylinder. For the sake of symmetry, the preferred location of the virtual center is chosen to be along the axis of the cylinder. The cameras are oriented so that their optical axes are inclined with respect to the axis of the cylinder. The position of the common virtual optical center along the cylinder axis and the inclination of each camera are selected such that the reflective field of view captured by each camera: (1) is maximized while excluding the camera itself, and (2) the apparatus captures a 360xc2x0 strip around the front direction of the front hemisphere. The reflective cylinder (or only its front end) is hollowed and an additional camera is placed inside the cylinder such that its optical center coincides with the common virtual optical center of the facial cameras. Furthermore, the optical axis of this axial camera is aligned with the cylinder axis, pointing toward the front end of the cylinder. The edges of the front end of the cylinder are sharpened so that the cross section of the cylinder material does not block any light and as a result the usual fields of the axial and facial cameras are continuous. The inclinations of the facial cameras can be adjusted, for example, to control the relative sizes of their field of view""s and the field of view of the axial camera while maintaining continuity across the fields of view captured by the facial and axial cameras. Together these facial and axial cameras provide images of contiguous parts of the front hemisphere, the relationships among these parts being adjustable. These cameras will be called front facial (or front side-view) and front axial cameras in the sequel. FIG. 1 illustrates this part of the apparatus and method of the current invention.
To accomplish the second part of the aforementioned and other objects, there is provided a second part of the apparatus in which extensions of the existing cylindrical faces are made or new faces are added (both called rear faces). Additional facial cameras (to be called rear facial cameras) are placed to image parts of the rear hemisphere by capturing the light reflected by the rear faces. The relative locations and orientations of the rear reflecting faces and the rear facial cameras are adjusted with respect to the existing reflective cylinder and front cameras so that the rear facial cameras and the front facial and axial cameras share a common virtual optical center, and the rear facial cameras capture light from specific parts of the rear field of view. Following examples illustrate embodiments of configurations of rear faces and rear facial cameras.
As example 1, a pair of a single rear camera and a single rear reflecting surface are included such that the reflecting surface (to be called rear reflecting face) redirects light from the rear axial direction into the added camera (to be called rear axial camera). The rear axial camera captures this redirected light to provide an image of the rear axial field of view, analogous to the image of the front axial field of view provided by the front axial camera. The rear reflecting surface and its associated rear camera may be configured with to avoid possible self occlusion by the rear camera.
As example 2, the single rear reflective surface of the example 1 is replaced by a right reflective pyramid with apex on the cylinder axis pointing in the rear direction. Thus the front side field of view is imaged using the cylinder faces and the rear side field of view is imaged using the pyramid faces. The facial cameras associated with the new rear side faces provide a contiguous image of the rear field of view about the rear axis.
As example 3, the pyramid of example 2 is replaced with a cylinder with polygonal cross section (to be called rear cylinder) which is rotated about the axis so its cross sectional edges are not parallel to the cross sectional edges of the front cylinder. Rear facial cameras are placed associated with the rear cylinder faces, analogous to the front faces and front facial camera, and they extend the front side field of view captured by the front facial cameras in the rear direction.
As a set of examples, the configurations of examples 1 and 3 are combined to simultaneously provide images of both rear side field of view and rear axial field of view.
The apparatus of the first and second parts are combined to obtain different embodiments that simultaneously provide panoramic images of both front and rear field of view.
Front and rear cameras of different resolutions are used to obtain different embodiments that provide panoramic images of different resolutions.
Front and rear cameras sensitive to different wavelengths are used to obtain different embodiments that provide panoramic images at different wavelengths (e.g., visible, infrared, etc.) FIGS. 1-9 Illustrate the apparatus and method of the current invention.